1. Field of the Invention
The invention relates to a method for digital generation of SAR images obtained by means of a coherent imaging system carried by a carrier, a signal compression being obtained in the azimuth and/or range direction with high resolution by a coherent summation of subapertures and an apparatus for carrying out said method.
2. Description of the Prior Art
Radar with synthetic aperture (SAR) is employed for two-dimensional imaging of the earth's surface, of planets and objects. Because it is independent of the weather, can be used during the day and during the night and has a high geometrical resolution, imaging with SAR is very advantageous. However, very complicated processing is necessary to generate an SAR image and at present only a few specially constructed computers achieve a real time processing with high image quality.
Hereinafter the mode of operation of an SAR system and the necessary data processing for generating an image of high resolution and quality will be explained. An SAR system includes a carrier, for example an aircraft, a helicopter, a satellite and the like, which moves with constant velocity, an antenna having a viewing direction aligned transversely, i.e. to the left or right, of the moving direction, and a coherent radar system which periodically transmits electromagnetic pulses. The backscattered radar echoes are received, mixed down and quadrature demodulated (in the I and Q channel), digitized and processed to generate a two-dimensional image. The movement direction is denoted as azimuth and the direction perpendicularly thereto as range; the bandwidth of the transmitted pulses defines the range resolution. A high range resolution is possible by linear frequency modulation (increasing the bandwidth). In this case, processing of the backscattered radar echo in the range direction is necessary to achieve a high range resolution. This processing is defined in accordance with the theory of the matched filter, a convolution of the received signal with the conjugated complex time-inverted replica of the transmitted modulated pulse being carried out. The replica is referred to as reference function, a convolution operation as pulse compression and a resultant function of the pulse compression from a single imaged point target as point target response.
The azimuth resolution for a conventional incoherent side-looking radar is limited by the physical antenna length in the aircraft. The azimuth resolution is therefore defined by the product of antenna aperture angle and distance between carrier and target. In this case it is therefore not possible to carry out and obtain imaging with high resolution from a great distance.
An azimuth resolution of synthetic aperture radar (SAR) is very much improved by using an antenna with wide beam in the azimuth and carrying out a coherent imaging. In this case a given target is illuminated by the antenna during several pulses and each echo is received coherently. A long synthetic antenna can be formed in that the quadratic phase variation in the azimuth direction caused by the range variation during the illumination time between carrier and target is corrected. To enable a phase information to be evaluated a coherent radar system is required for the transmission and reception.
A quadratic phase characteristic in the azimuth direction means a linear frequency modulation and this is referred to as Doppler characteristic. The entire bandwidth of an azimuth signal is referred to as Doppler bandwidth. To achieve high resolution, processing in the azimuth direction consists of convoluting the azimuth signal with a reference function calculated from the geometry and this is then referred to as pulse compression in the azimuth direction. Since the target illumination time in the azimuth direction increases linearly with the range, the length of the synthetic aperture becomes greater with increasing range. As a result, the azimuth resolution becomes independent of the range.
Image representation of processed complex data requires formation of an absolute value. To do this, the signals in the I and Q channel are squared and added (i.e. (I.sup.2 +Q.sup.2)); thereafter, the square root is taken therefrom.
If the range variation during the illumination time of the radar system in the azimuth direction is greater than half the resolution in the range direction, range migration must be corrected. A further processing is necessary in the azimuth processing when the Doppler centroid, i.e. the frequency in the centre of the Doppler bandwidth, is not equal to zero. In such a case the bandwidth of the reference function is adapted to the Doppler bandwidth of the azimuth signal by a frequency shift. To reduce the speckle noise present due to coherent processing in the imaging of area targets in SAR images, a socalled multi-look processing is carried out in the azimuth direction (see U.S. Pat. No. 4,292,634). In a multi-look processing an incoherent addition is carried out of statistically independent images which are generated by division of the available Doppler bandwidths into individual looks and generated by the conventional signal compression.
The standard deviation of speckle noise decreases with the square root of the number of looks, leading to an improved radiometric resolution and thus to better image quality. The image quality of an SAR image depends on the contrast, geometric and radiometric resolution, the side lobe suppression of the point target response and a loss in the processing. The looks may also overlap, leading to a more effective utilization of the bandwidth available. More looks can then be formed by the overlapping and thus a better radiometric resolution achieved. For this reason an overlapping of up to 60% is usually employed.
If the velocity of the carrier is not constant, small deviations of the nominal path of the carrier occur or the distance variation between carrier and target is not known, automatic focussing must be carried out in the azimuth processing to avoid the contrast and geometric resolution in the azimuth direction deteriorating (see EP 0 083 107 A3). An optimum reference function is then calculated from an analysis of the azimuth signal by automatic focussing. The calculation must be carried out accurately frequently enough to enable rapid variations of the movement error to be corrected. With the automatic focussing, not only the correct flying speed and its variations are determined but also phase errors resulting from horizontal and vertical deviations from the desired path.
Due to the high data rate, which is usually greater than 5 Mbyte/s, and due to a complicated data processing for generating an SAR image, consisting of a signal compression in the azimuth and range direction, a correction of the range migration a Doppler centroid determination, a multi-look processing and an automatic focussing, frequently more than 10.sup.10 operation/s are necessary to carry out a real-time processing.
In conventional SAR processors processing is carried out with digitized data and for this purpose computer systems are employed having array processors or special hardware configurations. In U.S. Pat. No. 4,132,989 (by R. A. Frosch and W. E. Arens of January 1979) a digital SAR processing in the time range is described; in this case a time correlation of a received signal with a reference function is formed in the azimuth and range direction. As soon as the number of points (P) of the reference function is more than 32 said method requires a high computing expenditure. This is because for each correlated point P complex multiplications and (P-1) complex additions are necessary. Other operations, such as range migration, are also carried out in the time range with very high computing expediture.
At present, a digital SAR processing in the frequency domain is mostly employed (see Wu, C.: "Digital Fast Correlation Approach SAR Imagery", Proc. of the IEEE Int. Radar Conf., p. 153 to 160 of April 1980). Here, a fast Fourier-transformation (FFT) algorithm is employed for a signal compression in the range and azimuth direction. This method is based on the fact that a convolution in the time domain in accordance with the convolution theory of the Fourier transformation corresponds to a multiplication in the frequency domain. The signal received and the reference function are first Fourier-transformed, then multiplied with each other and finally transformed back to the time domain by the inverse Fourier transformation. This processing is substantially faster than the formation of the time correlation and therefore also makes it possible to carry out the correction of the range migration and the Doppler centroid determination in efficient manner in the frequency domain.
A subaperture method is also employed for the digital SAR data processing. This method is based on the coherent addition of subapertures which are obtained by dividing the signal bandwidth in the azimuth and/or range direction. In
U.S. Pat. No. 4,227,194 the subapertures with adapted phase correction are coherently added, a linear phase correction being carried out in each subaperture by a fedback summation term.
M. Sack (see Sack, M. et al: "Application of Efficient Linear FM Matched Filtering Algorithmus to Synthetic Aperture Radar Processings", IEEE Proc., Vol. 132, No. 1, p. 45 to 57 of 1985) describes a digital method for SAR data processing by frequency analysis which is referred to as SPECAN method. In this method a received signal is mixed with a frequency-modulated signal having an opposite frequency characteristic. A frequency analysis is then carried out of the result of the mixing by means of a fast Fourier transformation (FFT). This method can be carried out faster than the method operating in the frequency range because only one fast Fourier transformation (FFT) is required for generating an image. Disadvantages of the SPECAN method are the sample spacing of the image data varying with the modulation rate, the inflexibility for correcting the quadratic range migration and the poorer geometric resolution.
To obviate the disadvantages of the SPECAN method and improve the image quality, a socalled step transform method has been developed (see Sack M. et al as above and Medina, M., Magota N.: "Implementation of a Subaperture Image Formation Technique for SAR", ISW '89, Eleventh Annual Ideas in Science and Electronics Exposition and Symposium, Editor: Christman C., Alubuquerque, NM, USA: Ideas in Science and Electronics 1989, pages 179 to 186). In the step transform method the mixing signal is divided into smaller overlapping signals and the fast Fourier transformation is replaced by two fast Fourier transformations with smaller number of points. By the first Fourier transformation the subapertures are formed and by the second Fourier transformation the subapertures are coherently summated.
To perform a frequency analysis of the SAR signal, multiphase filters, referred to as polyphase filters, are employed. In EP patent application 227 614 A2 the digital SAR processing with the polyphase filters is a modified version of the processing by frequency analysis. To accelerate the processing the polyphase filters are implemented by parallel hardware.
An analog SAR processing with socalled SAW components (Surface Acoustic Waves) is employed at present in many radar systems with a pulse compression in the range direction. The processing is carried out in the intermediate frequency (IF) plane prior to the demodulation in quadrature, the SAW component playing the part of a delay network in the pulse compression. For the analog signal compression in the azimuth direction charge-coupled means (CCDs) are employed. With such charge-coupled devices the analog azimuth signal is sampled in the base band and a time correlation of the azimuth signal performed with the aid of an azimuth reference function.
A disadvantage of the known methods for digital generation of SAR images is the high computing expenditure. Due to the high demands a real-time processing can be achieved only with considerable expenditure because hardware implementation always involves high complexity, high power consumption, large dimensions and high costs. In the case of analog processing the dynamic range and the signal/noise ratio is restricted to 50 dB at the most; moreover, the flexibility and accuracy of the processing are reduced.